1. Field of the Invention
This invention relates to a control system for a power converter, and more particularly relates to a control system for a power converter which is composed of self-turn-off devices such as, gate turn-off thyristors (hereafter, simply GTOs), and is connected to a power system or loads via transformers, which can prevent DC magnetization of the transformer.
2. Description of the Related Art
FIG. 7 shows a schematic diagram of a prior art control system for a power converter which is composed of GTOs (hereafter, called a self-commutated converter), and is used in a DC transmission system or a static vat compensator and so on.
In FIG. 7:
1 is a power system;
2 is a transformer for connecting a self-commutated converter 3 composed of GTOs and power system 1;
4 is a DC power source such as capacitors etc.;
5 is a current detector which measures the output current of self-commutated converter 3;
6 is a potential transformer which measures the voltage of power system 1;
7 is a control circuit which controls the system voltage according to a system voltage reference 51;
8 is a DC component detector which detects the DC component contained in the output current of self-commutated converter 3 measured by current detector 5;
9 is an adder which adds the output of control circuit 7 which is an instruction value for the output voltage of self-commutated converter 3, and the output of DC component detector 8;
10 is a pulse-width modulation (PWM) control circuit which adjusts the output voltage of self-commutated converter 3 by determining the firing timing of the GTOs in response to the output of adder 9; and
11 is a gate pulse amplifier circuit for generating gate pulses for GTOs in self-commutated converter 3.
In FIG. 7, PWM control circuit 10 determines the GTO firing pattern so that no DC component is contained in the output voltage of self-commutated converter 3. However, the actual output voltage takes a waveform which contains a DC component due to the variations in the characteristic of the GTO and the variations in the gate signal transmission time.
When the output voltage of self-commutated converter 3 contains a DC component, the core of transformer 2 is magnetized asymmetrically because the voltage time product per cycle applied to transformer 2 does not become "0". Thus, the excitation current increases, and the output current of self-commutated converter 3 becomes over-current. This leads the stop of operation of self-commutated converter 3 for protection thereof. In the worst case, this sometimes leads to damage to the devices which compose self-commutated converter 3. In the prior art circuit shown in FIG. 7, to prevent DC magnetization the following control is performed. That is, the output current of self-commutated converter 3 is detected by current detector 5 and the DC component generated in the course of DC magnetization is detected by DC component detector 8. PWM control is then executed by adding the detected DC component and the instruction value from control circuit 7. Therefore, the output voltage of self-commutated converter 3 is adjusted so that DC magnetization is eliminated.
The voltage of power system 1 is usually AC voltage. However, transient DC components may be included in the voltage of power system 1 such as when connecting the power capacitor or the transformer to power system 1. In prior art technology, while it is possible to correct the DC component outputted from self-commutated converter 3, the DC magnetization due to the DC component generated by power system 1 cannot be suppressed, because no correction due to the DC component of power system 1 side is made. Therefore, when a transient DC component is generated in power system 1, transformer 2 will be magnetized asymmetrically and lead to an over-current.